The present invention relates to an optical pick-up apparatus and an optical disk apparatus by which optical information is reproduced by irradiation of optical beams such as laser beams onto an optical information recording medium such as optical disk, or the like.
Conventional technologies related to the present invention will be described below.
In FIG. 16, luminous flux emitted from a laser beam source 1 passes through a beam splitter 2, enters into a collimator lens 3, and is emitted as parallel luminous flux. This luminous flux is limited to a predetermined luminous flux by a diaphragm, and enters into an objective lens 6. When parallel luminous flux enters into the objective lens 6, the objective lens 6 forms an optical spot image, having no aberration, onto a information recording surface 8 through a substrate 7 having a predetermined thickness (herein, t=0.6 mm).
The luminous flux, which is modulated by information pits and reflected on this information recording surface 8, returns to the beam splitter 2 through the objective lens 6 and the collimator lens 3, is separated from the optical path of the laser beam source 1, and enters into a receiving means 9. This receiving means 9 is composed of PIN photo-diodes which are divided into a plurality of elements, and a current which is proportional to the strength of the entered luminous flux, is outputted from each element. This current is sent to a detection circuit system, not shown in the drawing, and an information signal, a focus error signal and a track error signal are generated in this detection circuit system. The objective lens 6 is controlled by a two-dimensional actuator, not shown in the drawing, composed of a magnetic circuit, a coil, or the like, according to the focus error signal and the track error signal, so that the optical spot position is always focused on the information track.
A large numerical aperture (for example, NA 0.6) is used in such an optical pick-up apparatus so that the diameter of an optical spot, collected by the objective lens 6, is reduced. Accordingly, when the thickness of the substrate 7, which is placed in the converging luminous flux, deviates from a predetermined thickness, large spherical aberrations are generated.
Referring to FIG. 17, this aberration will be explained. When the thickness of the substrate is changed in the objective lens, which is optimized under the condition that NA is 0.6, the wavelength of the laser beam emitted from the laser beam source is 635 nm, the thickness of the substrate is 0.6 mm, and the refractive index is 1.58, then, the aberration is increased by 0.01 .lambda.rms per 0.01 mm deviation. When the thickness of the substrate deviates by .+-.0.07 mm, the aberration is increased by 0.07 .lambda.rms, and reaches the Marechal criterion which is a value within which reading and writing operations can normally be performed.
Accordingly, when a information recording medium, made of a 1.2 mm thickness substrate, is used for reproduction instead of a information recording medium which is made of a 0.6 mm thickness substrate, an objective lens 36 and a diaphragm 35 corresponding to the 1.2 mm thickness substrate are provided in addition to the objective lens 6 corresponding to a 0.6 mm thickness substrate in the two-dimensional actuator portion, and the objective lens and the diaphragm are switched to the objective lens 36 and the diaphragm 35 for reproduction.
Alternatively, the following is also considered. Two optical pick-up apparatus respectively used for an information recording medium made of a 0.6 mm thickness substrate and for an information recording medium made of a 1.2 mm thickness substrate, are provided in a single optical disk apparatus.
Further, the following is also considered. A hologram is provided in the optical pick-up apparatus, and a 0-order light and a 1st-order light which transmit the hologram, are respectively used as optical spots corresponding to the 0.6 mm thickness substrate and the 1.2 mm thickness substrate, and these optical spots are converged on the information recording surface.
As described above, in an apparatus in which two objective lenses respectively corresponding to 0.6 mm and 1.2 mm thickness substrates are provided, or two optical pick-up apparatus for the 0.6 mm and for 1.2 mm thickness substrates are used, in order to reproduce an information recording medium having different thickness substrates in one optical disk apparatus, it is impossible to produce a compact optical pick-up apparatus and an optical disk apparatus low in cost.
Further, even when the thickness of the substrate is the same, in cases of information recording mediums (for example, CDs, DVDs) in which difference of the image recording density is large, the size of the spot to be read is inappropriate for the information pit size, resulting in a decrease of reproduction performance.
In the method in which a hologram is provided in the optical pick-up apparatus, and a 0-order light and a 1st-order light which transmit the hologram, are respectively used as optical spots corresponding to the 0.6 mm thickness substrate and the 1.2 mm thickness substrate, and these optical spots are converged onto the information recording surface, since two luminous flux are always sent to the information recording surface of the information recording medium, one luminous flux is not necessary for information reading out when information reading out is carried out by the optical spot of another luminous flux, resulting in increase of noise. Further, since the laser beam strength is used under divided condition, the S/N ratio is decreased by a corresponding decrease of the amount of light, or the life of the laser beam is decreased as the amount of light is increased. Further, in the method in which recording is carried out, since a loss of light emission is increased, a higher output laser apparatus is required, resulting in a further increase of cost.
Further, in cases where track errors are detected by a 3-beam method, when the number of types of azimuth of 3 beams is one with respect to information tracks on the optical disk, track error signals can not be detected effectively with respect to both optical disks having different track pitches (for example, 0.84 .mu.m and 1.6 .mu.m), and thus the tracking performance is reduced.